Incidence and effects of anomalies and hybridization on Alabama freshwater fish index of biotic integrity results

The index of biotic integrity (IBI) is an effective multi-metric tool for assessing biological integrity of aquatic ecosystems and regulating water quality based on fish community surveys. Percentages of individuals with deformity, eroded fin, lesion, and tumor (DELT) anomalies are used in IBIs as a measure of fish health, while the percentage of hybrids reflects changes in reproductive isolation. When rare, these are combined into a joint ‘% DELT + hybrids’ metric. We investigated the distribution and effects of this metric on overall fish IBI scores and biological condition ratings by analyzing data from 646 fish surveys from across Alabama. Following exploratory data analyses, we assessed the impacts of % DELT + hybrids by comparing IBI scores for sites before and after setting this metric to zero to simulate systematic error (no detection). While DELTs and other anomalies were infrequent (~ 11% of sites), the two most commonly diseased fish species were Campostoma oligolepis (Leuciscidae) and Lepomis cyanellus (Centrarchidae). Hybrids were less frequent (~ 7% of sites) than anomalies and were scored only for Lepomis sunfishes. Overall, we found no statistically significant differences in final IBI scores or biological condition ratings before and after anomaly and hybrid removal; however, removal led to increased IBI scores and biological condition ratings in 13.2% and ~ 6% of surveys, respectively. These findings demonstrate that, while IBI outcomes appear relatively robust to systematic error in % DELT + hybrids, anomaly and hybrid observations can alter IBI outcomes, with important implications for fish-based bioassessments and water quality management.